Fix the keybindings to follow GNU conventions

Since 'C-c *letter*' are reserved for users, all such keybindings have
been changed to 'C-c C-*letter*', except

1) 'el-elisp-macroexpand-all' - This has been rebound to 'C-c M-m'
2) 'el-toggle-context-help' - This rebound to 'C-c C-?' ('C-h' has not
    been used, since it is used by emacs built-in help system)

The README has been updated to reflect the changes, this fixes #3